1. Field of the Invention
The present invention relates to a process of manufacturing a semiconductor device and, more particularly, to a process of forming a conductive film having improved coverability characteristics.
2. Description of the Prior Art
To electrically connect a conductive film to a semiconductor substrate or to another conductive film in an integrated circuit, a portion of an insulating film isolating the conductive film from the semiconductor substrate or other conductive film is removed to form a contact window. The improvement of the step coverage of the conductive film in the contact window is essential to improving the reliability of the integrated circuit.
FIGS. 3a to 3c show different stages of a conventional process of manufacturing a conductive film.
Referring to FIG. 3a, an SiO.sub.2 (silicon dioxide) film 12 is formed over the surface of a silicon substrate 11 by using a thermal oxidation process. A conductive film 13 is formed over the surface of the SiO.sub.2 film 12 by using a sputtering process. The conductive film 13 is, for example, an aluminum film or a polycrystalline silicon film. An insulating film 14 having a thickness of 8000 A is formed over the surface of the conductive layer 13 by using a CVD chemical vapor deposition) process. The insulating film 14 is, for example, a phosphorus silica glass (PSG) film. A photoresist pattern 15 is formed over the surface of the insulating film 14 by using a photolithographic process.
Referring to FIG. 3b, a portion of the insulating film 14 is removed by performing a reactive ion etching (RIE) process using the photoresist pattern 15 as a mask to form a contact window 16, and then the photoresist pattern 15 is removed.
Referring to FIG. 3c, a second conductive film 17 having a thickness of 7,000 A is formed over the outer surface of the structure obtained in the preceding process by performing a sputtering process. The thickness of the second conductive film 17 is less in portions near the edge 14a of the insulating film 14 due to a shadow effect produced by the edge 14a. The shadow effect is described in detail in James F Smith, "Influence of DC bias Sputtering During Aluminum Metallization", Solid State Technology, pp. 135-138, January 1984.
As an improperly thin conductive film has an excessively high current density, it is subject to electromigration. The second conductive film 17 is often broken by such electromigration.
In another known process of forming a conductive film, the edge 14a of the insulating film 14 is rounded as shown in FIG. 4 to obviate such a problem. Referring to FIG. 4, the silicon substrate 11 is annealed at a temperature in the range of 800.degree. to 1000.degree. C. before causing the second conductive film 17 to melt and round the edge 14a, and then the second conductive film 17 is formed over the surface of the structure formed in the preceding process by using a sputtering process. This process requires a material having a high melting point, such as polycrystalline silicon, for forming the first conductive film 13.
The latter known process is comparatively more effective at improving the step coverage of the second conductive film 17; however, ions of an impurity implanted in the silicon substrate 11 are diffused excessively resulting in a deterioration of the characteristics of the transistor of the integrated circuit.